KVM: arm64: Save/Restore guest DISR_EL1

If we deliver a virtual SError to the guest, the guest may defer it
with an ESB instruction. The guest reads the deferred value via DISR_EL1,
but the guests view of DISR_EL1 is re-mapped to VDISR_EL2 when HCR_EL2.AMO
is set.

Add the KVM code to save/restore VDISR_EL2, and make it accessible to
userspace as DISR_EL1.

Signed-off-by: James Morse <james.morse@arm.com>